Polyester resins have found widespread use in varied applications. Polyesters such as polyethylene terephthalate are used in photographic film, in magnetic tape, in fibers, and in food and beverage containers. Various methods have been disclosed for the depolymerization of such resins into their component monomers, such as ethylene glycol and terephthalic acid or derivatives thereof, so that they can be reused.
Some of these methods are described in such patents as U.S. Pat. Nos. 3,037,050, 3,321,510, 3,884,850, 3,907,868, 4,163,860, 4,578,502, 4,620,032, 4,876,378 and 5,095,145, and in European Published Patent Application 0 484 963 published May 13, 1992.
A particularly useful technique for recovering scrap polyester is described in a series of patent that begins with Naujokas et al. U.S. Pat. No. 5,051,528. This patent describes a process of recovering ethylene glycol and dimethyl terephthalate from polyethylene terephthalate scrap resins by dissolving the polyester resin in oligomers of the same monomers as are present in the polyester, passing super-heated methanol through the solution and recovering ethylene glycol and dimethyl terephthalate.
Gamble et al. U.S. Pat. No. 5,298,530, issued Mar. 29, 1994 improves on the process of the '528 patent by combining scrap resin with reactor melt in a dissolver before the dissolver melt is transferred to the reactor for contact with super-heated methanol. In the reactor, polymers and oligomers are further depolymerized into the component glycol and ester monomers, which are then recovered.
Toot et al. U.S. Pat. No. 5,414,022, issued May 9, 1995, optimizes the conditions of the processes of Naujokas et al. and Gamble et al., cited above.
DeBruin et al U.S. Pat. No. 5,432,203, issued Jul. 11, 1995, extends the processes of prior patents in the series to convert ethylene glycol and dimethyl terephthalate to bishydroxyethyl terephthalate, which then can be used as feedstock for the formation of polyethylene terephthalate.
The processes described in this series of patents and applications have numerous advantages. These include low cost, high efficiency, the ability to operate at relatively low pressure and the ability to be used with a variety of forms of polyester of varying degrees of cleanliness and purity.
The processes and equipment described in this series of patents and applications employ a reactor in which a discontinuous phase of superheated methanol is passed through a continuous phase of molten polyester and polyester decomposition products. While such a reactor is useful, we have found that the conversion rate of polyester to monomer can be improved by the use of a reactor in which the superheated methanol is the continuous phase and molten polyester and polyester decomposition products are the discontinuous phase. We have found that this can be accomplished by using a staged column as the reactor.